React Patterns

React Development Patterns

This skill provides expert guidance on modern React development patterns and best practices.

When to Apply This Skill

Use these patterns when:

• Creating or refactoring React components
• Implementing state management with hooks
• Optimizing React performance
• Designing component architecture
• Working with React 18+ features

Core Principles

1. Component Design

Functional Components First

• Always prefer functional components over class components
• Use hooks for state and side effects
• Keep components focused and single-purpose

Component Composition

// Good: Composable, reusable
function UserProfile({ user }) {
  return (
    <Card>
      <Avatar src={user.avatar} />
      <UserInfo name={user.name} email={user.email} />
      <UserActions userId={user.id} />
    </Card>
  );
}

// Avoid: Monolithic, hard to test
function UserProfile({ user }) {
  return (
    <div>
      {/* Everything in one component */}
    </div>
  );
}

2. Hook Patterns

State Management

• Use useState for simple local state
• Use useReducer for complex state logic
• Use useContext for shared state (sparingly)

Effect Patterns

// Good: Proper dependency array
useEffect(() => {
  fetchUser(userId);
}, [userId]);

// Good: Cleanup for subscriptions
useEffect(() => {
  const subscription = api.subscribe(topic);
  return () => subscription.unsubscribe();
}, [topic]);

Custom Hooks for Reusable Logic

function useDebounce(value, delay) {
  const [debouncedValue, setDebouncedValue] = useState(value);

  useEffect(() => {
    const handler = setTimeout(() => setDebouncedValue(value), delay);
    return () => clearTimeout(handler);
  }, [value, delay]);

  return debouncedValue;
}

3. Advanced State Management

When your app grows beyond simple local state, use external state management libraries.

Zustand (Recommended for Most Projects)

Why Zustand:

• Minimal boilerplate
• No providers needed
• TypeScript-friendly
• Built-in devtools support
• Middleware for persistence, immer, etc.

Installation:

npm install zustand

Basic Store:

// stores/userStore.ts
import { create } from 'zustand';

interface User {
  id: string;
  name: string;
  email: string;
}

interface UserState {
  user: User | null;
  isLoading: boolean;
  error: string | null;

  // Actions
  setUser: (user: User) => void;
  fetchUser: (id: string) => Promise<void>;
  logout: () => void;
}

export const useUserStore = create<UserState>((set) => ({
  user: null,
  isLoading: false,
  error: null,

  setUser: (user) => set({ user }),

  fetchUser: async (id) => {
    set({ isLoading: true, error: null });
    try {
      const response = await fetch(`/api/users/${id}`);
      const user = await response.json();
      set({ user, isLoading: false });
    } catch (error) {
      set({ error: error.message, isLoading: false });
    }
  },

  logout: () => set({ user: null }),
}));

Using the Store:

function UserProfile() {
  // Subscribe to specific state slices
  const user = useUserStore((state) => state.user);
  const fetchUser = useUserStore((state) => state.fetchUser);
  const isLoading = useUserStore((state) => state.isLoading);

  useEffect(() => {
    fetchUser('123');
  }, [fetchUser]);

  if (isLoading) return <Skeleton />;
  return <div>{user?.name}</div>;
}

// Outside components
function logoutEverywhere() {
  useUserStore.getState().logout();
}

Zustand with Immer (for complex state):

import { create } from 'zustand';
import { immer } from 'zustand/middleware/immer';

interface TodoState {
  todos: Todo[];
  addTodo: (text: string) => void;
  toggleTodo: (id: string) => void;
}

export const useTodoStore = create<TodoState>()(
  immer((set) => ({
    todos: [],

    addTodo: (text) =>
      set((state) => {
        // Mutate draft directly (Immer handles immutability)
        state.todos.push({
          id: crypto.randomUUID(),
          text,
          completed: false,
        });
      }),

    toggleTodo: (id) =>
      set((state) => {
        const todo = state.todos.find((t) => t.id === id);
        if (todo) todo.completed = !todo.completed;
      }),
  }))
);

Zustand with Persistence:

import { create } from 'zustand';
import { persist } from 'zustand/middleware';

export const useSettingsStore = create<SettingsState>()(
  persist(
    (set) => ({
      theme: 'light',
      language: 'en',
      setTheme: (theme) => set({ theme }),
      setLanguage: (language) => set({ language }),
    }),
    {
      name: 'app-settings', // localStorage key
    }
  )
);

Zustand Slices Pattern (for large stores):

// stores/slices/userSlice.ts
export const createUserSlice = (set) => ({
  user: null,
  setUser: (user) => set({ user }),
});

// stores/slices/settingsSlice.ts
export const createSettingsSlice = (set) => ({
  theme: 'light',
  setTheme: (theme) => set({ theme }),
});

// stores/index.ts
import { create } from 'zustand';

export const useStore = create((set) => ({
  ...createUserSlice(set),
  ...createSettingsSlice(set),
}));

Redux Toolkit (for Enterprise Apps)

When to use Redux Toolkit:

• Large enterprise applications
• Need time-travel debugging
• Complex state logic with multiple reducers
• Team already familiar with Redux

Installation:

npm install @reduxjs/toolkit react-redux

Store Setup:

// store/userSlice.ts
import { createSlice, createAsyncThunk } from '@reduxjs/toolkit';

export const fetchUser = createAsyncThunk(
  'user/fetch',
  async (userId: string) => {
    const response = await fetch(`/api/users/${userId}`);
    return response.json();
  }
);

const userSlice = createSlice({
  name: 'user',
  initialState: {
    data: null,
    loading: false,
    error: null,
  },
  reducers: {
    logout: (state) => {
      state.data = null;
    },
  },
  extraReducers: (builder) => {
    builder
      .addCase(fetchUser.pending, (state) => {
        state.loading = true;
      })
      .addCase(fetchUser.fulfilled, (state, action) => {
        state.loading = false;
        state.data = action.payload;
      })
      .addCase(fetchUser.rejected, (state, action) => {
        state.loading = false;
        state.error = action.error.message;
      });
  },
});

export const { logout } = userSlice.actions;
export default userSlice.reducer;

// store/index.ts
import { configureStore } from '@reduxjs/toolkit';
import userReducer from './userSlice';

export const store = configureStore({
  reducer: {
    user: userReducer,
  },
});

export type RootState = ReturnType<typeof store.getState>;
export type AppDispatch = typeof store.dispatch;

Using Redux:

// hooks/redux.ts
import { useDispatch, useSelector } from 'react-redux';
import type { RootState, AppDispatch } from '../store';

export const useAppDispatch = useDispatch.withTypes<AppDispatch>();
export const useAppSelector = useSelector.withTypes<RootState>();

// Component
function UserProfile() {
  const dispatch = useAppDispatch();
  const { data: user, loading } = useAppSelector((state) => state.user);

  useEffect(() => {
    dispatch(fetchUser('123'));
  }, [dispatch]);

  return <div>{user?.name}</div>;
}

Jotai (Atomic State)

When to use Jotai:

• Bottom-up state management approach
• Need derived state
• Want something lighter than Redux but more structured than Zustand

Installation:

npm install jotai

Basic Atoms:

// atoms/user.ts
import { atom } from 'jotai';

export const userAtom = atom<User | null>(null);
export const isLoadingAtom = atom(false);

// Derived atom
export const userNameAtom = atom(
  (get) => get(userAtom)?.name ?? 'Guest'
);

// Async atom
export const userDataAtom = atom(
  async (get) => {
    const response = await fetch('/api/user');
    return response.json();
  }
);

Using Atoms:

import { useAtom, useAtomValue, useSetAtom } from 'jotai';

function UserProfile() {
  const [user, setUser] = useAtom(userAtom);
  const userName = useAtomValue(userNameAtom); // Read-only
  const setLoading = useSetAtom(isLoadingAtom); // Write-only

  return <div>{userName}</div>;
}

State Management Decision Tree

Do you need global state?
├─ No → Use useState/useReducer
└─ Yes
   ├─ Simple global state (settings, user, etc.)
   │  └─ Use Zustand
   ├─ Complex enterprise app with time-travel debugging
   │  └─ Use Redux Toolkit
   └─ Need fine-grained reactivity and derived state
      └─ Use Jotai

State Management Best Practices

1. Collocate state - Keep state as close as possible to where it's used
2. Server state vs Client state - Use React Query/TanStack Query for server state
3. Don't over-globalize - Not everything needs to be in a store
4. Use selectors - Only subscribe to the state you need
5. Normalize complex data - Avoid deeply nested state structures

Example: Combining Zustand with React Query

// Server state with React Query
const { data: todos } = useQuery({
  queryKey: ['todos'],
  queryFn: fetchTodos,
});

// Client state with Zustand
const filter = useStore((state) => state.filter);
const setFilter = useStore((state) => state.setFilter);

// Combine for derived state
const filteredTodos = useMemo(
  () => todos?.filter(todo =>
    filter === 'all' || (filter === 'completed' && todo.completed)
  ),
  [todos, filter]
);

4. Performance Optimization

Memoization Strategy

• Use useMemo for expensive calculations
• Use useCallback for function references passed to children
• Use React.memo for expensive components that render often

When to Optimize

// Optimize when rendering is expensive
const ExpensiveList = React.memo(({ items }) => {
  const sortedItems = useMemo(
    () => items.sort((a, b) => a.priority - b.priority),
    [items]
  );

  return sortedItems.map(item => <Item key={item.id} {...item} />);
});

4. Props and TypeScript

Prop Design

• Keep prop interfaces simple and focused
• Use TypeScript for type safety
• Avoid prop drilling (use composition or context)

interface ButtonProps {
  variant: 'primary' | 'secondary' | 'danger';
  size?: 'sm' | 'md' | 'lg';
  onClick: () => void;
  children: React.ReactNode;
  disabled?: boolean;
}

function Button({ variant, size = 'md', onClick, children, disabled }: ButtonProps) {
  // Implementation
}

5. Project Structure

Recommended Organization (with Zustand)

src/
├── components/
│   ├── common/          # Reusable UI components
│   ├── features/        # Feature-specific components
│   └── layouts/         # Layout components
├── stores/              # Zustand stores
│   ├── userStore.ts
│   ├── settingsStore.ts
│   └── slices/          # Store slices for large apps
├── hooks/               # Custom hooks
├── contexts/            # Context providers (use sparingly)
├── utils/               # Helper functions
└── types/               # TypeScript types

Common Patterns

Container/Presentational Pattern

// Presentational: Pure UI
function UserCard({ name, email, onEdit }) {
  return (
    <div>
      <h3>{name}</h3>
      <p>{email}</p>
      <button
    </div>
  );
}

// Container: Logic and data
function UserCardContainer({ userId }) {
  const [user, setUser] = useState(null);

  useEffect(() => {
    fetchUser(userId).then(setUser);
  }, [userId]);

  const handleEdit = () => {
    // Edit logic
  };

  return user ? <UserCard {...user} /> : <Skeleton />;
}

Compound Components Pattern

function Select({ children, value, onChange }) {
  return (
    <select value={value}
      {children}
    </select>
  );
}

Select.Option = function Option({ value, children }) {
  return <option value={value}>{children}</option>;
};

// Usage
<Select value={country}
  <Select.Option value="us">United States</Select.Option>
  <Select.Option value="ca">Canada</Select.Option>
</Select>

Best Practices

1. Always use keys in lists - Use stable, unique IDs
2. Avoid inline object/array literals in props - Causes unnecessary re-renders
3. Extract complex JSX into variables - Improves readability
4. Use error boundaries - Catch and handle component errors gracefully
5. Lazy load routes and heavy components - Use React.lazy() and Suspense

Code Review Checklist

When reviewing React code, check for:

• Proper use of hooks (no hooks in conditionals/loops)
• Correct dependency arrays in useEffect/useMemo/useCallback
• No unnecessary re-renders (use React DevTools Profiler)
• Proper key props in mapped lists
• TypeScript types for all props
• Accessibility (semantic HTML, ARIA attributes)
• Error handling and loading states

Anti-Patterns to Avoid

❌ Mutating state directly

// Wrong
state.items.push(newItem);
setState(state);

// Correct
setState({ ...state, items: [...state.items, newItem] });

❌ Using index as key in dynamic lists

// Wrong
{items.map((item, index) => <Item key={index} {...item} />)}

// Correct
{items.map(item => <Item key={item.id} {...item} />)}

❌ Over-using useEffect

// Wrong: Derived state in effect
const [fullName, setFullName] = useState('');
useEffect(() => {
  setFullName(`${firstName} ${lastName}`);
}, [firstName, lastName]);

// Correct: Calculate during render
const fullName = `${firstName} ${lastName}`;

Resources

• React documentation: https://react.dev
• React TypeScript Cheatsheet: https://react-typescript-cheatsheet.netlify.app
• React Performance: https://react.dev/learn/render-and-commit